MIL-DTL-5794C
4.6 Test procedures.
4.6.1 Visual and mechanical inspection. The microphone unit shall be given a thorough mechanical
inspection for size, loose or defective parts, workmanship, visible defects, marking, weight and
dimensions.
4.6.2 Response tests.
4.6.2.1 Sound pressure meter. The sound pressure meter specified on figure 4, or equal, shall be
used in setting up sound levels for performing the response measurements specified herein. The
microphone of this sound level meter shall be a Western Electric 640-A or 640-AA condenser microphone
or a demonstrated equivalent. The sound pressure meter microphone shall be calibrated by the
reciprocity method in accordance with ANSI S1.15 (Part 2) at a recognized laboratory.
4.6.2.2 Sound source calibration. The equipment and test setup shown on figure 5, or its specifically
approved equivalent, shall be used to provide the sound input to the microphone under test for all
response measurements. The test fixture (see Drawing SC-D-10598) employed to hold the microphone
unit under test shall be so constructed as to permit rotation of the microphone unit when mounted in the
fixture (for conditioning purposes), and shall support the microphone unit in such a fashion that the grid
area of the microphone unit (with no surrounding baffle) is exposed to the sound source. The grid of the
microphone unit under test shall be spaced 0.25 (¼) inch from the mouth of the sound source. The
geometric centerline of the grid shall coincide with the centerline of the mouth of the sound source. The
input to the drive unit required to produce a sound pressure of 115 dB above a reference level of 20 µPa,
at the grid of the microphone unit under test, shall be determined for each 100 Hz increment from 200 to
1,000 Hz inclusive, each 200 Hz increment from 1,000 to 4,000 Hz inclusive, and each 500 Hz increment
from 4,000 to 5,000 Hz inclusive. The grid of the 640-A condenser microphone shall be spaced 0.25 (¼)
inch from the mouth of the sound source during calibration, and the centerline of the sound source mouth
and the 640-A condenser microphone shall coincide. No baffle shall be used with the 640-A condenser
microphone. At each frequency the distortion in the output of the sound source shall be such that the
level of the second harmonic is at least 35 dB below the fundamental and the levels of the third and
higher harmonics are each at least 40 dB below the fundamental.
4.6.2.3 Conditioning of microphone unit. Immediately prior to response measurements, each
microphone unit shall be conditioned using the test circuit on figure 3, according to the following
procedure.
4.6.2.3.1 Secure microphone in test fixture. The microphone unit shall be secured to the test fixture,
and electrically connected to Jack J-1 of the test circuit. The microphone shall be shaken vigorously in all
directions before mounting it in the test fixture.
4.6.2.3.2 Adjust for microphone current. Switch S-1 of the test circuit shall be closed (no microphone
current) and resistor R-1 shall be adjusted to produce a reading of 60 milliamperes for M-6A/UR, and 85
milliamperes for M-51/UR, on meter M-1.
4.6.2.3.3 Rotate microphone. The microphone unit shall be rotated about its axis 3 times, through an
arc of approximately 270 degrees and back in 2 seconds. The microphone unit shall be in the position at
which it is to be tested at the completion of this rotation procedure.
4.6.2.3.4 Apply conditioning sweep tone. Switch S-1 shall be opened, approximately 3 seconds shall
be allowed for current stabilization, then the conditioning sweep tone specified in 4.6.2.3.5 and 4.6.2.3.6
shall be applied. The voltage of the conditioning sweep tone shall have been previously determined as
follows:
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